1. Field of the Invention
The present invention relates to engine bearings. More particularly the present invention relates to an improved sealing arrangement for an oil lubricated bearing used with a turbine.
2. Description of the Prior Art
The use of oil lubricated bearings in turbines is well known. For example, U.S. Pat. Nos. 3,722,624, issued to Buckland on Mar. 27, 1973, 3,779,345, issued to Barnes et al on Dec. 18, 1973, and 4,046,223, issued to McHugh on Sept. 6, 1977, all of which are assigned to the assignee of the present invention, show such lubricated bearing systems. The subject matter of U.S. Pat. No. 4,046,223 is hereby incorporated by reference.
A typical known form of prior art structure is shown in FIG. 1, wherein an oil lubricated bearing 10 (shown in cross section) rotatably supports shaft 12 of a turbine. The bearing 10, which includes a bearing housing 14 and bearing liner 16, is fed oil or similar lubricating fluid by way of sump 22. The forward end 24A, as viewed in the drawing, of bearing 10 includes a seal having an air seal portion 26A and an oil seal portion 28A which serve to minimize leakage of oil from the bearing. A similar seal having an air seal portion 26B and an oil seal portion 28B minimizes leakage on the rear side 24B of bearing 10. As is well known in the art, compressor discharge air cooperates with the seals to minimize oil leakage from the bearing 10. Air which has leaked past the air seal portion of one of the seals may proceed out of vent 30 by way of a leakage path such as 32A or 32B. Oil which has leaked past one of the oil seal portions may proceed through one of the leakage paths such as 32C or 32D. Such leakage oil will then accumulate in the lower portion of the seal air cavities, 34A and 34B. The oil in cavities 34A and 34B will flow through drain-back holes 36A and 36B back to oil drain flow path in sump 22 and drain pipe 20.
One problem with the simple drain-back holes is that they eventually become blocked due to varnish build-up from the leaking lube oil. Varnish build-up is caused by the high service temperature of the metal in which the drain-back holes are drilled. The high metal temperature in turn is brought about by the fact that some bearing housings operate in hot ambient environments. The bearing housing of a gas turbine, for example, is surrounded by compressor discharge air which may exceed 550.degree. F.
When the drain-back holes are blocked and the turbine is operated with a momentary loss of seal air, leakage oil can accumulate in the seal cavity between the oil and air seals (FIG. 1). Continued leakage may cause the leakage lube oil to spill past the air seals and outside the bearing housing. An accumulation of oil outside the bearing housing poses a serious fire hazard.
It should also be noted that turbines usually rely on their own high speed rotation to generate seal air, therefore, are frequently placed in a situation in which seal air is absent, the lube oil pumps are on, and the bearing housings are hot from prior turbine operation. Turning gear operation is the most prominent example of this situation. Thus the conditions favoring a blockage of the drain-back holes are not unusual.
Three choices suggest themselves for resolving the drain-clogging problem, each having certain disadvantages.
1. Stop the lube oil pumps when seal air is absent. This approach suffers from the disadvantage that the lube oil flow at shutdown is essential to keep heat soak-back from damaging the low melting point bearing liner babbitt. PA1 2. Provide an auxiliary source of seal air not dependent upon high speed rotation of the turbine itself. This approach requires additional equipment with the consequent disadvantages of high cost, lower reliability and high power consumption. PA1 3. Enlarge the drain-back hole size. This has been the standard response to resolve the problem. The objective of this approach is to lengthen the time it takes for varnish or sludge to plug up the hole; however, increased hot gas leakage occurs during normal operation, because the drain-back hole provides a direct by-pass around the labyrinth seal. Increased seal air leakage is of course undesirable from a power loss standpoint.
It is therefore an object of the present invention to provide an improved sealing arrangement to minimize leakage of oil, or similar lubricating fluids, from a bearing.
A further and more specific object of the present invention is to provide an arrangement which avoids clogging of the leakage oil drain holes in a turbine journal bearing.